Vibratory meters, such as, for example, vibrating densitometers and Coriolis flow meters are generally known and are used to measure mass flow and other information for materials within a conduit. The vibratory meter comprises a sensor assembly and an electronics segment. The material within the sensor assembly may be flowing or stationary. Each type of sensor may have unique characteristics, which a vibratory meter must account for in order to achieve optimum performance.
Exemplary Coriolis flow meters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450 all to J. E. Smith et al. The sensor assemblies in the Coriolis flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in the sensor assembly has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode. Material flows into the sensor assembly from a connected pipeline on the inlet side of the sensor, is directed through the conduit(s), and exits the sensor through the outlet side of the sensor. The natural vibration modes of the vibrating material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the sensor assembly, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or small “zero offset,” which is a time delay measured at zero flow. As material begins to flow through the sensor assembly, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the sensor lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
Meter electronics connected to the driver generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pick-off sensors. The driver may comprise one of many well known arrangements; however, a magnet and an opposing drive coil have received great success in the flow meter industry. An alternating current is passed to the drive coil for vibrating the conduit(s) at a desired flow tube amplitude and frequency. It is also known in the art to provide the pick-off sensors as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current which induces a motion, the pick-off sensors can use the motion provided by the driver to induce a voltage.
The vibratory meters are used in many applications, including custody transfer. Custody transfer typically involves transferring a batch of material from seller to buyer in, for example, a tank. An example of a custody transfer is fuel bunkering. Bunkering refers to the practice of storing and transferring marine fuel oils, which have come to be known as bunker fuels. Bunker fuel comprises a relatively heavy petroleum derivative that is used in heating or in large industrial and/or marine engines. Bunker fuel is generally heavier and more viscous than gasoline or diesel.
For ship fueling, large amounts of fuel may be temporarily stored in a barge or other container for the purpose of transferring fuel from shore to a ship. A bunker may be located on a dock or other port facility, or may be carried by a barge or other refueling vehicle. During bunkering, the fuel measurement usually comprises an empty-full-empty batching process. This empty-full-empty batching process can cause gas to become entrained in the fuel.
Improvements in vibratory meters have made it possible to obtain more accurate measurements of fuel, even when the fuel has entrained gas. However, a problem can exist whenever flow is stopped, for example, at the beginning or at the end of the bunkering process due to a change in the zero offset of the vibrating meter. Even after fuel has stopped flowing through the vibratory meter, the flow tubes continue to vibrate. Ideally, the time delay between the pick-off sensors would return to the original zero offset value when the flow through the tubes is zero. As long as the time delay returns to the original zero offset, the vibratory meter will report a zero mass flow. However, various factors attribute to the zero offset of the sensor assembly and some of the factors may change either during the bunkering process or after the last zeroing process.
For example, while many vibratory meters are capable of maintaining accurate measurements despite entrained gas, in some situations when the flow through the flow tubes falls to zero, the entrained gas can lead to an imbalance that creates asymmetric damping between the inlet and the outlet side of the vibrating meter's sensor assembly. The asymmetric damping can cause a time delay between pick-offs, which may be different than the original zero offset and thus may be interpreted as real flow. This problem may also be experienced if, for example, the sensor assembly is only partially filled with fluid, for example, which may occur during bunkering.
Accordingly, there is a need for detecting an inaccurate flow rate measurement by a vibratory meter. There is also a need to provide methods and apparatuses for detecting inaccurate flow rate measurements with existing installed flow meters and without additional installed hardware.